1. Field of the Invention
This invention relates generally to active solid-state devices, and more particularly to a transient voltage suppression device having one or more avalanche diodes.
2. Description of the Related Art
Transient voltage suppression (“TVS”) devices comprising an avalanche diode are well known. As the reverse avalanche voltage is made smaller, a depletion region of the avalanche diode narrows, resulting in a higher internal capacitance of the avalanche diode. As operating frequencies become higher, the internal capacitance of the avalanche diode becomes problematic. A known solution to the capacitance problem is to add a rectifier diode in series with the avalanche diode, with either the anodes or the cathodes of the diodes connected together. A rectifier diode has a smaller capacitance than an avalanche diode, and the total capacitance of a pair of such diodes in series is less than the sum of the two capacitances.
TVS devices having both diodes of such pair on a single die are also known. For example, U.S. Pat. No. 6,392,266 entitled TRANSIENT SUPPRESSING DEVICE AND METHOD, issued May 21, 2002, to Robb et al., discloses two transient voltage suppressors that are housed in a single semiconductor package, each transient voltage suppressor comprising two serially coupled diodes on one die. A TVS device comprising such a pair of diodes is a unidirectional device in that the TVS device provides protection against voltage spikes or surges in one direction only.
Bi-directional TVS devices comprising two such pairs of diodes in an anti-parallel configuration are also known. Known bi-directional TVS devices comprise at least two die, wire bonded together inside a single semiconductor package. One example of such a TVS device is the Model No. PSLC03 thru PSLC24C family of TVS devices manufactured by ProTek Devices of Phoenix, Ariz., which includes four die inside a single semiconductor package. Such TVS devices work well for their intended uses, but when a very small bi-directional TVS device is required, a TVS device comprising a single die is preferred.
The reverse avalanche voltage, or breakdown voltage, is defined as the voltage at which the avalanche diode goes into avalanche mode, measured at a relatively low current such as one milliamp. The breakdown voltage is controlled by the doping level of an N+ diffusion layer relative to the doping level of a P+ diffusion layer of the avalanche diode. The clamping voltage is defined as the maximum voltage across the TVS device when a maximum surge current is flowing through it. The clamping voltage is typically measured at a relatively high current such as one amp. As a result, the clamping voltage is normally higher than the breakdown voltage. The clamping voltage of a TVS device is directly, although not necessarily linearly, proportional to the breakdown voltage of the avalanche diode. The amount by which the clamping voltage is greater than the breakdown voltage is directly proportional to the geometry of the PN junction and to the diffusion depth of the avalanche diode. A higher background resistivity of a doped epitaxial region of the die results in a higher clamping voltage relative to the breakdown voltage.
As electronic devices, especially battery-operated portable electronic devices such as cellular telephones become smaller, there is a need for a smaller TVS device. It is desirable that a TVS device have as low a clamping voltage as possible. When the TVS device reaches its clamping voltage, the TVS device prevents the electronic device under protection from exposure to any higher voltage than the clamping voltage. The clamping voltage of a prior art TVS device would disadvantageously rise if the avalanche diode were simply made smaller because a smaller PN junction area has a higher resistance.